The ability to coat tablets well is often considered more of an art than a science. Traditional coating uses a sugar solution that is applied one layer at a time through a process called the three D’s; dose, distribute, and dry. Once at the proper temperature the tumbling cores are exposed to an initial dose of solution which is added by the use of a (soup) ladle, pumped, or a controlled spray nozzle. The solution is added in steps and after each addition the cores are continually tumbled to distribute the solution evenly while forced air is blown over the surface of the bed of cores to facilitate the drying process. This process of dose, distribute, and dry is done layer by layer until the required 50-100% weight gain achieves the desired buildup and appearance. Many companies made their own proprietary solution that was kept a closely guarded secret. The art was knowing when the cores were ready for the next layer. If solution was added too soon the cores would stick to each other pulling the coating off other cores. If the previous coating layer was over-dried then the next layer wouldn’t adhere properly creating blemishes and rough surfaces, resulting in a defective coating. In the old days coating a tablet was an art, having a lot of touch and feel. Today coatings are demanding because they must do more than a sugar coat and taste mask. Nowadays most tablets are coated with a thin film, referred to as film coating. A typical film is either just a thin clear coating or a more controlled build up of color coating, most commonly a 3% build up of total tablet weight.

1. hen it comes to coating tablets there are
musts that really count and then there are
the other things.
W The ability to coat tablets well is often
considered more of an art than a science. Traditional coat-
ing uses a sugar solution that is applied one layer at a time
through a process called the three D’s; dose, distribute, and
dry. Once at the proper temperature the tumbling cores
are exposed to an initial dose of solution which is added by
the use of a (soup) ladle, pumped, or a controlled spray
nozzle. The solution is added in steps and after each ad-
dition the cores are continually tumbled to distribute the
solution evenly while forced air is blown over the surface of
the bed of cores to facilitate the drying process. This process
of dose, distribute, and dry is done layer by layer until the
required 50-100%weight gain achieves the desired buildup
and appearance. Many companies made their own
proprietary solution that was kept a closely guarded secret.
The art was knowing when the cores were ready for the next
layer. If solution was added too soon the cores would stick to
each other pulling the coating off other cores. If the previous
coating layer was over-dried then the next layer wouldn’t
adhere properly creating blemishes and rough sur- faces,
resulting in a defective coating. In the old days coat- ing a
tablet was an art, having a lot of touch and feel. Today
coatings are demanding because they must do more than a
sugar coat and taste mask. Nowadays most tablets
are coated with a thin film, referred to as film
coating. A typical film is either just a thin clear
coating or a more controlled build
up of color coating, most commonly a
3% build up of total tablet weight.
Film coatings provide much more
than just being a sweet taste and a
protective layer. From a manufactur-
ing standpoint a film coating elimi-
nates dust, makes the tablet tougher,
and improves packaging capabilities.
From the users standpoint it improves
stability, increased tablet hardness,
can contain a drug layer, controls the re-
lease rates, and improves the appearance.
The process starts with tumbling to elimi-
nate the dust. Dust can create poor adhesion of
the initial layer preventing the solution from
impinging into the surface of the tablet which would easily
peel away regardless of how many layers were added. If the
first coat isn’t good no amount of subsequent layers will
overcome a weak foundation. Once dedusted the known
volume of cores (tablets) are loaded into a drum called a
coating pan to create a batch of product. The cores are
exposed to treated and conditioned air, this air stabilizes the
temperature of the cores and the coating chamber to enable
accurate adhesion of a sprayed solution. The more we control
these conditions the more likely the process will become a
predictive one.
Film coating is not at all like sugar coating in that as soon
as the solution (dose ) hits the tablet in film coating it
should dry immediately. The prospect of spreading the
solution across the surface like sugar coating would cause
damage to the tablet. To make the art of coating more scien-
tific, predictable, and most importantly repeatable here are
key elements.
M u s t s
Tablet Hardness: The greatest influence of the success in
film coating is the consistency of tablet hardness. The
delivery of a sprayed coating solution onto a variable range
of tablet surface hardness will net poor results at best. Soft
tablets will often result in a dull coating and risk the poten-
tial for erosion. Tablets that are too hard don’t allow the
solution to lock into the surface of the tablet and the
coating will often peel off. These are extreme
examples, but the subtle variations in tablet
hardness can also cause picking, mottling,
and reduce the tablet strength. If Iwere
an inventor Iwould want a tablet sur-
face hardness detector for the best
coating results. Traditional hardness
testing is not as predictable when it
comes to a film coating. s t a r t u p :
Load: Be careful to load tablets and
look for broken, chipped, and other de-
fects as the tablets are being loaded. Be
sure to position the pan so the tablets do
not hit the mixing baffles or anything else
that could dame or chip them. One broken or
chipped tablet can ruin the entire batch. Also
tablets need to settle after compression for at
Tablet Coating Musts
A quick look at what you must know to be successful
■By Mike Tousey, Techceuticals
■26 m a y 2014 | P h a r m a c e u t i c a l P r o c e s s i n g
■p h a r M p r o . c o M
■c o a t i n g
Tablet cores are too soft, tablets
breaks apart too easily

2. least 12-24hours or the consistency of coating will not be
predictable and may peel or frost.
Dedust: Turn on the exhaust blower and pull the excess dust
off the tablets. Jog the pan at a very slow speed and make sure
that tablet flash or extrusions are removed with the dust.
Preheat: When it comes to film coating the tablets must be
preheated or the coating will not dry immediately. The sweet
spot is generally around 43-45°C. Dropping below 42and going
over 48°C is asking for problems (there are always ex-
ceptions). Remember that when the spray cycle begins it will
have a cooling effect. If temperatures are too high the coating
may flash off and not impinge into the surface.
Avoid Black Marks: Sometimes it is helpful to preheat the
coating pan before loading tablets that get black marks from the
stainless steel. Many calcium based products mark very easily.
To avoid the marks there are two methods; preheat the pan or
coat the pan with the same solution used to coat the tablets.
Pan RPM: Initially the pan should be jogged to dedust, do
not run the pan continuously prior to turning the spray on to
avoid erosion. Tablet shape and bed depth have an im- pact on
the flow of the cores within the pan. Generally start up slow at
around 4-5 rpm once the initial coating is applied then speed
can and should be increased. In general the ob- jective is a
tranquil bed of tablet cores, we do not want to see tablets
jumping all over the place.
s t a r t s p r a y i n g
Coating Solution: Prepare the coating solution ahead of time
to allow the suspension to adequately dissolve. This means the
solution should be prepared a minimum of 60min- utes before
hand. Make certain the solution is free of lumps and air
bubbles. Most solutions require continuous mixing throughout
the process. Again the name of the game is repro- ducibility
batch to batch. Establishing a solution hold time is a good idea;
and the ideal range is typically 2-24hours. The wider the range
often times the less predictable the results. Spray too soon and
the guns clog easily, wait too long and the viscosity changes
due to evaporation. The hold time is depen- dent on the type of
solution, water or solvent. The window for solvents based
coating solutions needs to be narrower.
Solution Spray Rate: The objective is to spray a thin film
across the surface of the tablet bed to get even coverage a
little at a time. We want small amounts of coverage multiple
times, not the other way around. The spray rate depends on
the solution and the core. Generally solution rates are
between 40-120ml/minute per gun. The spray should dry on
contact. Too much air pressure will cause spray drying,
orange peel, and poor coverage. While not enough air will
cause over wetting resulting in sticking, picking, peeling and
worse. Again the key is consistency.
Drying Air: The coating pan has small holes to allow air to
dry the coating solution. The air supply and exhaust pressure is
balanced to minimize the rubbing of the tablets against the
perforated pan surface. If supply air was only blown into the pan
the spray would go everywhere. If exhaust air was only pulled
out of the pan the air pressure would damage the tab- lets. The
proper way air is balanced is that the incoming air is pushed
along with the current of the sprayed solution while at the same
time the air is pulled through the tablets like pulling air through
a filter. The air flow needs to be fast to carry the moisture away
from the tablets. Solvent solutions are not as reliant on air
volume as aqueous based solutions.
Build Up: Most film coatings quantities are visual or a thin
3%buildup by weight. Typically tablets are weighed after
they are preheated and then the weights are checked
throughout the process until the desire weight gain is
achieved or the appearance is achieved. Many companies
predetermine the quantity of solution to be added and sim-
ply spray until it is all gone.
Cool Down: Once the proper coating is on the core the spray
is turned off. When the spray is turned off the heat should be
turned off. Slow the pan rpm or put into a jog mode until the
tablets are cooled and dried. Unload and inspect carefully.
t h i n g s c a n g o W r o n g :
Bearding: Dried solution occurs from the spray drying
around the nozzle. This can happen because of a poorly pre-
pared solution, improper spray rates, too much atomization air
pressure, and in-
adequate negative
pan pressure.
Broken
Solution Lines:
If a solution line
breaks shut off
the spray and
the heat supply,
keep the pan
tumbling to pre-
vent sticking.
Make the repairs
and continue the
process.
Poor Negative Pressure: Make certain the dust collector
can handle the entire run. If the exhaust air volume and static
pressure change this can cause over wetting and ulti- mately
create many defects.
P h a r m a c e u t i c a l P r o c e s s i n g | m a y 2014 27■
■p h a r M p r o . c o M
■c o a t i n g
Solution adhesion can cause coatingdefects
Coating solution must be mixed thoroughly